Communication networks increasingly rely upon optical fiber for high-speed, low-cost transmission. Optical fibers were originally envisioned as an optical replacement for electronic transmission media, such as high-speed coaxial cable and lower-speed twisted-pair cable. However, even high-speed optical fibers are limited by the electronics at the transmitting and receiving ends, generally rated at a few gigabits per second, although 40 Gb/s systems have been prototyped. Such high-speed electronic systems are expensive and still do not fully exploit the inherent bandwidth of fiber-optic systems, measured in many terabits per second.
All-optical transmission systems offer many intrinsic advantages over systems that use electronics within any part of the principal transmission path. Wavelength-division multiplexing (WDM) electronically impresses different data signals upon different carrier frequencies, all of which are carried by a single optical fiber. The earliest WDM systems did not provide optical switching but only point-to-point WDM.
Recent research and development have suggested that an all-optical network can be constructed having switching nodes that can switch the separate WDM channels (carrier frequencies) in different directions without the necessity of converting the optical signals to electronic signals. If such optical switching can be accomplished with simple optical components, a sophisticated optical network can be constructed at relatively low cost with the high-speed electronics being confined to end terminals that require speeds of only the individual channels and not of the total throughput of the system.
However, such optical switching needs to effectively separate the switched channels. A cross-talk requirement of 20 dB is a minimum, 35 dB would be a reasonable design requirement, 40 dB would be better. Also, the switching bands should be relatively wide to accommodate significant frequency fluctuations in the optical transmitters, particularly due to frequency chirping in directly modulated laser sources. That is, the switch must have its frequency bands registered with the transmitter even when the transmitting frequency is varying somewhat. The combination of a wide switching band and low cross talk requires a flat-top switch spectrum. Furthermore, a somewhat minimal WDM switch has a size of 2.sup.4 .times.2.sup.4, that is, two physical input fibers and two output fibers, each bearing four WDM channels freely switched from either input to either output.
Cheung et al. in U.S. Pat. No. 5,002,349 have suggested that an acousto-optical tunable filter (AOTF) be used in such a WDM network, either at the switching node or at the terminal end. However, AOTFs have many intrinsic problems, such as cross-talk between adjacent-frequency signal. To date, these problems have prevented ATOFs from being adopted into communication networks. The physical mechanisms of AOTFs seem to preclude a good flat-top response.
Patel, sometimes in conjunction with co-inventors, has suggested that liquid-crystal filters be used in such WDM communication networks; see, for example, U.S. Pat. Nos. 5,111,321 and 5,150,236. Indeed, Patel has suggested in U.S. Pat. No. 5,111,321 that a liquid-crystal system could be used as a drop-add circuit. However, such a system appears difficult to implement.
Weiner and collaborators have disclosed how an optical signal can have its frequency-divided components separately phase-modulated or amplitude-modulated by using a diffraction grating to divide the input signal into spatially separated frequency components which are separately operated upon by a segmented modulator. See, for example, U.S. Pat. No. 4,685,547 to Heritage et al. Patel et al. have applied this concept to a system incorporating liquid-crystal modulators, as disclosed in U.S. Pat. No. 5,132,824.
The use of diffraction gratings for multiplexing in a WDM system has been described by Nishi et al. in "Broad-passband-width optical filter for multi/demultiplexer using a diffraction grating and a retroreflector prism," Electronics Letters, vol. 21, 1985, pp. 423-424 and by Shirasaki et al. in "Broadening of bandwidths in grating multiplexer by original dispersion-dividing prism," Electronics Letters, vol. 22, 1986, pp. 764-765.
Nonetheless, the prior art fails to disclose an effective, economical optical switch for a WDM telecommunication system.